1. Field of the Invention
The present invention relates generally to an apparatus and method for generating a high-order harmonic X-ray, and a point-diffraction interferometer using the high-order harmonic X-ray, and more particularly to an apparatus and method for generating a high-order harmonic X-ray source with excellent coherence and realizing a point-diffraction interferometer which can inspect a surface quality of an optical system for extreme ultraviolet lithography using the high-order harmonic X-ray source.
2. Description of the Prior Art
Generally, an interferometer is a device for dividing light emitted from a single light source into two or more light beams in an appropriate manner, overlapping divided light beams to cause the light beams to interfere with one another, and observing an interference pattern of the interfered light beams. An interferometer was mainly used to measure wavelengths, precisely compare lengths or distances, compare optical distances, and perform other functions. In recent years, the utilization range of the interferometer has been widened to applications for inspecting a surface quality of an optical system or the like.
There are a “point-diffraction interferometer using an X-ray generated in a synchrotron”, a “point-diffraction interferometer using a white light” or the like as the interferometer used for the above purpose.
The point-diffraction interferometer using an X-ray generated in a synchrotron employs an X-ray generated in the synchrotron as a light source, so the point-diffraction interferometer must use a synchrotron so as to generate the light source. However, such point-diffraction interferometer is problematic in that the access to a huge synchrotron (with the size of several hundreds of meters) is limited and it is difficult to inspect a great number of optical systems for industrial use.
Meanwhile, in an interferometer used for inspection or measurement, a light source must be coherent. However, the above point-diffraction interferometer using an X-ray generated in synchrotron is problematic in that additional devices are required to improve the coherence of the light source, thus complicating the entire construction and operation of a measuring device and greatly weakening the intensity of light reaching a sample from a light source.
Meanwhile, a point-diffraction interferometer, in which a pinhole 1 for improving coherence is disposed between a light source and a radiation optical system 3, and a beam splitter 2 for dividing a beam into two separate beams is disposed between the pinhole 1 and the radiation optical system 3, is disclosed in U.S. Pat. No. 5,835,217 by Medecki, et al., entitled a “Phase-shifting point-diffraction interferometer”, as shown in FIG. 1.
This apparatus is advantageous in that it uses a white light passing through the pinhole 1, so a short coherence length of the white light is sufficiently utilized to obtain a good interference pattern 5, while it is disadvantageous in that it requires the pinhole 1 which is an additional device for improving the coherence of a light source as described above. A similar circumstance holds for the case of a point diffraction interferometer that uses X-ray from a synchrotron.